Integrated circuits (ICs) are widely used in electronic devices. Integrated circuits are typically formed on a semiconductor wafer using photolithographic processes and then packaged, for example in a ceramic or plastic package, with pins or bumps on the package providing externally accessible electrical connections to the integrated circuit. An unpackaged integrated circuit is often referred to as a die. Each die typically has electrical contact pads on the top of the integrated circuit that are electrically connected to electronic circuits in the integrated circuit. The die is placed in a cavity in the package, the electrical contact pads are wire-bonded to the package pins or bumps, and the package is sealed. Frequently, multiple identical devices are formed in the semiconductor wafer and the wafer is cut (for example by scribing-and-breaking or by sawing the wafer) into separate integrated circuit dies that are each individually packaged. The packages are then mounted and electrically connected on a printed circuit board to make an electronic system.
In an alternative flip-chip approach, small spheres of solder (solder bumps) are deposited on the integrated circuit contact pads and the integrated circuit is flipped over so that the top side of the die with the solder bumps is located adjacent to the package or other destination substrate. This approach is particularly useful for packages such as pin-grid array packages because they can require less space than a wire-bond process. However, flipping the integrated circuit over can be difficult for very small integrated circuits, for example having a dimension of less than 200, less than 50 microns, less than 20 microns, less than 10 microns, or less than 5 microns. Such small integrated circuit dies are not easily handled without loss or damage using conventional pick-and-place or vacuum tools.
In some applications, the bare integrated circuit dies are not separately packaged but are placed on a destination substrate and electrically connected on the destination substrate, for example using photolithographic or printed-circuit board methods, to form an electronic system. However, as with flip-chip handling, this can be difficult to accomplish when the integrated circuit dies are small. However, an efficient method of transferring bare dies from a relatively small and expensive source substrate (e.g., crystalline semiconductor) to a relatively large and inexpensive destination substrate (e.g., amorphous glass or plastic) is very desirable, since the integrated circuits can provide much higher data processing efficiency than thin-film semiconductor structures formed on large substrates.
In another method, a handle substrate is adhered to the side of the integrated circuits opposite the wafer (the top side), the wafer is removed, for example by grinding, the integrated circuits are adhered to the destination substrate, and the handle substrate is removed. In yet another variation, the handle substrate is the destination substrate and is not removed. In this case, the integrated circuit is flipped over so that the top side of the integrated circuit is adhered to the destination substrate.
In yet another method, epitaxial semiconductor layers are formed on a growth substrate, for example a sapphire substrate. A handle substrate is adhered to the top side of the semiconductor layers opposite the growth substrate, and the growth substrate is removed. The flipped semiconductor layers are then processed to form the integrated circuits. For example, U.S. Pat. No. 6,825,559 describes such a method to make light emitting diodes.
One approach to handling and placing small integrated circuits (e.g., chiplets) uses micro-transfer printing, for example as described in U.S. Pat. Nos. 8,722,458, 7,622,367 and 8,506,867, each of which is hereby incorporated by reference in its entirety. In exemplary embodiments of these methods for printing integrated circuits, an integrated circuit is disposed (e.g., formed) on a source wafer, for example a semiconductor wafer, and undercut by etching a gap between a bottom side of the integrated circuit and the wafer. A stamp contacts a top side of the integrated circuit to adhere the integrated circuit to the stamp, the stamp and integrated circuit are transported to a destination substrate, for example a glass or plastic substrate, the integrated circuit is contacted and adhered to the destination substrate, and the stamp removed to “print” the integrated circuit from the source wafer to the destination substrate. Multiple integrated circuits can be “printed” in a common step with a single stamp. The integrated circuits can then be electrically connected, for example, using conventional photolithographic and printed-circuit board methods. These techniques have the advantage of enabling location of many (e.g., tens of thousands to millions) small integrated circuit devices on a destination substrate in a single print step. For example, U.S. Pat. No. 8,722,458 teaches, inter alia, transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate using a patterned elastomer stamp whose spatial pattern matches the location of the semiconductor elements on the wafer substrate.
None of the aforementioned flip-chip methods form a flipped integrated circuit that can be micro-transfer printed. Moreover, GaN micro-LEDs are typically formed on sapphire substrates since sapphire has a smaller crystal lattice mismatch with GaN than other materials, such as silicon. Thus, it is desirable to form printable integrated circuit structures, such as micro-LEDs, using a sapphire substrate. However, there is no available method for undercutting a chiplet formed on a sapphire substrate to enable release of the chiplet for micro-transfer printing.
There is a need, therefore, for wafer and integrated circuit structures and methods that provide flipped micro-transfer printable integrated circuits and for structures and methods that enable the construction of micro-LED chiplets formed on a sapphire substrate that can be micro-transfer printed. There is also a need for simple and inexpensive methods and structures having a reduced area that enable electrical interconnections for chiplets printed on destination substrates. Furthermore, there is a need for methods and structures that enable electrically connecting the electrical contacts of printed structures, such as printed LEDs, using fewer processing steps than conventional methods.